The construction industry relies on materials testing for design, quality control and quality assurance of various construction projects. Material density and specific gravity are among some of the critical parameters of materials testing. The pavement construction industry, in particular, uses material density and specific gravity in the design and quality determinations of natural and manufactured paving materials.
In the asphalt paving industry, air void contents of hot-mix asphalt laboratory prepared specimens or cored pavement specimens are used, for example, to determine the quality of the mix design, the plant-produced hot-mix, and the pavement construction. The air void content of compacted specimens is determined, in some instances, as a ratio of the actual specific gravity of the compacted specimen (bulk specific gravity) to the theoretical maximum specific gravity of the loose asphalt mixture.
The determination of the maximum specific gravity or density of the loose asphalt mixtures may have some limitations that affect the accuracy of the air void content measurement. Furthermore, methods of determining bulk specific gravity are highly operator dependent and therefore may yield highly variable results, also affecting the air void content determination. Currently, there are three generally accepted practices or methods of determining the bulk specific gravities of compacted asphalt specimens. These methods are (1) gamma attenuation; (2) applications of Archimedes' principle; and (3) dimensional analysis.
Gamma attenuation technology can be used to provide bulk density of a compacted asphalt specimen by measuring its electron density as described, for example, in U.S. Pat. No. 6,492,641 to Dep et al. The electron density is determined by the intensity and energy distribution of gamma radiation traversing the sample. The gamma radiation is typically emitted from low-level radiation cesium sources and detected by a sensitive sodium iodide detector. The resulting measurement of the electron density must then be normalized by the height (or thickness) of the specimen. However, while the electron density determination is generally precise and reliable, the gamma attenuation method may be limited by the ability of the operator to measure the height of the specimen with accuracy and precision.
ASTM D3549 is a standard test method for thickness or height determination of compacted bituminous paving mixture specimens. The standard specifies that an average of four measurements, spaced apart at 90° intervals, should be used to approximate the height of the specimen. It further suggests that ends of the specimen that are not horizontal relative to the vertical axis of the cylinder shall be sawn flat and horizontal. However, there are several problems associated with this method. For example, in some cases, the operator may not ensure that the ends of the specimen are flat and horizontal, thereby introducing error into the height measurement because the end-to-end (or peak-to-peak) caliper measurements will not be reliable height measurements of the specimen. In such instances, the root-mean-square height may be a more accurate measure of the specimen height for density determinations. Another source of error in such a height measurement is that four measurements with the calipers may not provide enough data points to properly represent the true sample height, especially if the specimen is not a true right cylinder and/or if the ends thereof are irregular or sloped. Even if the operator uses extreme care and diligence in measuring the specimen height with the calipers, the calipers are not necessarily capable of properly measuring the irregular or uneven surfaces.
One widely used method of determining the bulk specific gravity of an asphalt mix specimen is by determining the mass to volume ratio of the specimen. Mass determinations are generally highly reliable through the use of state of the art balances and scales that are readily available in the marketplace. The volume measurement, however, is typically far less reliable than the mass determination. Several different methods of volume measurement incorporate the Archimedes' principal of water displacement. Another method of obtaining a volume measurement utilizes a dimensional analysis approach with calipers or micrometers.
The Archimedes' principal approximates the volume of a solid by determining the volume of water displaced by the solid when the solid is submerged in an adequately sized water bath. Generally, the ratio of the mass of water displaced to the specific gravity of the water is the resulting volume of the solid. However, in some instances, the determined volume may be adversely affected by water seeping into interconnected voids within the solid. In addition, the density of water is not constant and may be affected by temperature, impurities, or even an inconsistent water source. Consequently, the true volume of the solid may be an illusory quantity affecting the accuracy of the determined specific gravity and density of the solid, as well as the amount of water that is able to seep into the solid. However, another issue with the water displacement method is that submerging the sample in water is a destructive process. Though the sample may be dried after immersion, even very careful drying procedures do not typically provide repeatable specific gravity determination results for that sample in subsequent tests. The damage thus done to the specimen generally prohibits the use thereof in other material testing procedures.
Several AASHTO or ASTM standards utilize this water displacement principal in the determination of bulk specific gravity of compacted asphalt mixtures. However, basically all of these methodologies include inherent sources of error, typically depending on the conditions under which the procedures are performed. The saturated surface dry (SSD) method (AASHTO T166/ASTM D2726) tends to underestimate the volume of the specimen, thereby overestimating its bulk specific gravity or density. In order to overcome the limitations associated with the SSD method, techniques have been introduced that require coating the specimen with paraffin or parafilm (AASHTO T275/ASTM D1188), or vacuum sealing the specimen inside a plastic or poly-material bag(s) (ASTM D6752) as described, for example, in U.S. Pat. No. 6,321,589 to Regimand. However, these methods may overestimate the specimen volume by bridging the surface voids of the specimen, thus providing a resulting bulk specific gravity that is often lower than the true value of bulk specific gravity for that specimen. In addition, such methods may also require correction for the mass and volume of the coating or vacuum sealing bag, which may also introduce errors into the calculations.
The dimensional analysis method for determining the bulk specific gravity of the specimen approximates the volume by physically measuring the height and diameter dimensions of the specimen with calipers or micrometers. The specific gravity determined by the dimensional analysis method, however, is typically lower than the specific gravity determined by the water displacement method since dimensional analysis using calipers or a micrometer does not consider surface voids or other irregular surface features of the specimen.
In light of these limitations in being able to reliably determine the specimen height or other dimensions using existing technologies, there exists a need for a more reliable method for providing accurate dimensional values for a specimen. A method and/or apparatus is also needed that reduces the effect of operator judgment in determining specimen height or other dimensions so that single-laboratory and/or multi-laboratory variations do not affect the evaluations of the asphalt mix specimens. In addition, such an apparatus and/or method should be capable of nondestructively evaluating the specimen.